The major goal of the proposed research is to analyze the neurochemical substrates underlying pair bonding formation and social attachment in a monogamous rodent, the prairie vole (Microtus ochrogaster). Pair bonding formation is a behavioral characteristic of monogamous mammals such as prairie voles, and is rare in polygynous species such as montane voles (M. montanus) and meadow voles (M. pennsylvanicus) (19). The development of partner preferences and selective aggression toward strangers is used here as an operational definition of a pair bonding. Previous research has implicated arginine vasopressin (AVP) in the appearance of both partner preferences and selective aggression in male prairie voles (45). However, such effects have only been examined in males, and the specific brain areas in which AVP has influences on pair bonding have not been identified, since AVP was infused into the lateral ventricle. Sex differences exist in the expression of the behavioral characteristics of monogamy in the central AVP pathways, and mating-induced activity of these pathways in prairie voles (2,3,38). The propose research will compare male and female prairie voles to examine their behavioral differences in the effects of AVP in specific brain areas.